1. Field of the Invention
The present invention relates to the operation of pipets for picking up and dispensing predetermined volumes of fluid and, more particularly, to a pipet signalling arrangement which detects and signals the fluid dispensing action of the pipet.
2. Description of the Prior Art
Pipets are widely used in laboratory and clinical procedures which require a predetermined fluid volume to be aspirated from one vessel and the same or a different fluid volume to be dispensed into another vessel. Such pipets typically include a plunger or piston which is actuated in one direction to aspirate or draw fluid into an open end of the pipet and in the opposite direction to discharge fluid from the open end. In a common form, such pipets are sized to fit in an operator's hand and include a plunger actuator controlled by the operator's thumb or finger to drive the plunger in one direction and a return spring to drive the plunger in the opposite direction. The operator depresses the plunger actuator to drive the plunger forwardly against the spring force and expel air from the open end. The open end then is immersed in fluid and the plunger retracted rearwardly by the spring drawing fluid into the open end. Next, the pipet is positioned over or in a receiving vessel, and the plunger is again driven forwardly to expel fluid from the pipet into the vessel. The pipet is withdrawn from the vessel and the plunger retracted rearwardly to prepare for the next pickup operation. Usual pipet action thus comprises two successive plunger strokes--a fluid pickup stroke followed by a fluid dispensing stroke--each stroke comprising plunger movement in forward and rearward directions. In some laboratory procudures, the pipet tip is immersed in fluid (e.g. reagent) in the receiving vessel, and the fluid dispensing stroke comprises repeated actuation of the plunger back and forth to thoroughly mix the reagent and the dispensed fluid by the turbulent intake and discharge of both.
There are numerous pipetting applications in which it is desired to generate a signal at the time fluid is ejected or dispensed from the pipet. For example, in blood plasma prothrombin time determinations and in kinetic rate analysis of antigen-antibody reactions, it is necessary to measure the time interval from the beginning of a chemical reaction until the reaction reaches a predetermined condition. Typically, a reaction is initiated at the time of and by the act of pipetting the last reaction component into a reaction vessel or chamber. In such applications, a convenient manner of generating a control signal coincident with the dispensing of the last component is to incorporate a triggering switch o a set of electrical contacts within the pipet which are engaged or activated during the forward dispensing movement of the pipet plunger. While such arrangements are effective to signal the dispensing action of the pipet, unfortunately, they also operate to signal the pickup action since the plunger also executes forward (and rearward) movement during the pickup operation. As a result, during a fluid pickup and dispensing cycle, the pipet signalling arrangement generates two "dispensing" signals, namely, a first but false "dispense" signal during the pickup stroke and a second true "dispense" signal during the actual dispensing stroke.
The foregoing first and second pipe "dispensing" signals are distinguished, in one prior approach, by means of a second switching device which is actuated by the positioning of the pipet for a dispensing operation. In this respect, the second switch is typically disposed at the mount of the receiving vessel in a position to be engaged and actuated by the pipet body at the time the pipet is positioned over the receiving vessel. Of course, the pickup stroke is executed before so positioning the pipet. With the second switch actuated by positioning of the pipet, the dispensing stroke is executed to thereby additionally actuate the plunger controlled switch. The plunger actuated switch and the aforesdescribed second switch are coupled to logic circuitry which signals that a dispensing operation has occurred only if the plunger actuated switch is actuated during a period when the second switch is actuated.
While the above-dual switch approach is effective to signal fluid dispensing, it has a number of drawbacks. First, undue system complexity results by adding the additional switching device and logic circuitry for monitoring the switch signals. Moreover, the pipet is only capable of signalling a dispensing operation if used to dispense into a vessel or instrument in which such a second switch has been installed.
As a result, there is a need for a pipet signalling arrangement operative to detect and signal dispensing action of the pipet without the limitations of the prior art.